1. Field of the Invention
The invention relates processes for forming granulates from powders and/or particles, especially inorganic powders and/or particles, and organic polymers. The process can be operated continuously, and the resulting granulates are suitable for a variety of uses, particularly in purification of fluids, and even more particularly in the filtration of water and air.
2. Description of Related Art
In many areas of technology, it is often desirable to contact fluids with solid materials in order to separate materials in the fluid. For example, in the area of water purification, it is often desirable to separate undesirable volatile organic compounds (VOC), microorganisms, heavy metals, and other species from the water by contacting the water containing these contaminants with a solid material capable of absorbing or adsorbing them from the water. However, many of these solid materials, while capable of absorbing or adsorbing contaminants, are undesirable for use in purification applications because they can, over time, leach or otherwise deposit material into the fluid being purified.
This is particularly problematic when the adsorbing or absorbing material is a fine particulate (a characteristic that is very desirable from the standpoint of adsorption or absorption efficiency, since the smaller the particle, the more surface area is available for adsorption or absorption). Deposition of small particulates into the fluid can be particularity detrimental, since these particulates may contain relatively large amounts of the contaminant desired to be removed in the first place.
Additionally, the adsorbing or absorbing materials may themselves be considered to be contaminant in the fluid if they become entrained therein. This can occur by a variety of mechanisms, such as erosion of the adsorbing or absorbing material by contact with the fluid, or by the presence of small particles of adsorbing or absorbing material that can be present when the material is fire contacted with the fluid (i.e., dusting).
A number of different techniques have been developed in attempts to provide good separation efficiency while reducing contamination of the fluid by the separation medium. These include forming the separation material into porous blocks of particles held together with some form of binder resin. However, doing so sacrifices much of the advantages of high surface area that lead to the use of particulates in the first place. In addition, melting and extruding some HDPE binders into relatively large blocks makes uniform heating difficult, requiring the use of relatively high temperatures to provide adequate heat transfer to melt the binder material near the center of the block. This, in turn, may lead to hot spots in the material, in particular as the size of the block increases. The development of hot spots thus limits the size of the blocks that may be produced.
Another technique includes packing particles of the separation medium into canisters designed to allow fluid to flow in and out and to contact the separation medium, but to retain the particles within the coaster. The need to prevent escape of the particles from the canister places an effective lower limit on the size of the particles that can be used, and therefore of the surface area that can be obtained, and typically is not able to prevent introduction of some particulates into the fluid as the result of dusting of the separation medium.
Accordingly, there remains a need in the art for a material that can serve as a fluid separation medium, that contains fine particulates to provide good contact between the fluid and the separation medium, and that provides little or no leaching and/or dusting of the separation medium.
This invention satisfies these needs. It relates to a process for forming granules (and to the granules formed by the process) containing inorganic particles agglomerated with a binder containing a polymeric material, typically a high-density polyethylene (HDPE). The granules typically have sizes in the range from about 1 mesh to about 200 mesh.
The granules are formed by gradual heating of binder particles to a temperature below the melting point of the binder, but sufficiently above room temperature to soften the binder particles. The binder particles are combined with the inorganic particles and the two particles are mixed. This combination and mixing can occur prior to, during, or after heating of the binder particles, or some combination thereof. For example, in one embodiment of the invention, the binder particles are slowly preheated to a temperature below their melting temperature, and then combined and mixed with the inorganic particles, optionally with additional heat wherein the shear stress of mixing and/or additional heating is desirably sufficient to further soften and tackify the binder. In another embodiment of the invention, binder particles at ambient temperature are mixed and compacted with the inorganic particles, and the resulting mixture is heated to soften and tackify the binder particles.
The softened binder and the inorganic particles are then cooled. The result of this cooling may be used as the desired granules, or may be in the form of flakes or chips, that can be ground to form the desired granules of an appropriate size. Grinding of compressed material may also occur prior to heating, but after mixing and compaction of the binder and inorganic particles.
Without wishing to be bound by any theory, it is believed that during the initial heating the binder particles soften and become tact. At least a portion of the tacky surfaces of the softened binder particles comes into contact with the inorganic particles during mixing, and the inorganic particles adhere to the surfaces of the binder particles. During mixing and/or heating, at least some of the binder particles become further softened. Combined with optional applied compression, this draws the inorganic particles together and causes them to adhere to each other. The result is an agglomeration of the inorganic particles into granules, along with possibly the formation of binder particles having inorganic particles on their surface.
In embodiments of the invention where the binder particles are not preheated, it is believed that a first agglomeration of the inorganic particles with the binder particles occurs as the result of pressure compaction. The resulting agglomerates may be ground prior to heating, or may be heated without grinding, or may be ground after heating. The grinding is generally to a particle size of about 28 to about 48 mesh. In any case, the material is heated after compacting in an over to slowly raise the temperature of the material to below the melting point of the binder resin. Again, it is believed that the heating step renders the binder particles soft, tacky, and flowable, increasing adhesion to the inorganic particles.
A number of different methods exist for optionally preheating, and for combining and heating the HDPE binder and the inorganic particles that will form the desired granules, and these methods are within the scope of the present invention. These methods have in common the mixing of inorganic particles with the binder particles, and the subsequent heating and/or compression of the mixture to soften the binder and force the particles and the binder together, and the subsequent cooling of the heated material to form the desired agglomerated granules. The precise mechanisms and equipment used to carry out these processes may differ to some extent, but processes that carry out these steps are within the scope of the invention.
One suitable method for making the granules involves heating and mixing the binder and the inorganic particles in a twin screw compounder, which can be co-rotating or counter-rotating. In this embodiment, it is generally desirable to preheat the binder particles to a temperature sufficiently above ambient to render their surfaces tacky, but below the melting temperature. The binder can be heated by the application of external heat to make it tacky, and the inorganic particles added and combined with the tacky binder particles by the mixing action of the twin screws. Additional heating of the mixture may be provided by shear stress (as in an extruder) or by the application of external heating, or both. This additional heating raises the temperature of the binder sufficiently to cause further softening. The twin screws also desirably compact the mixture of inorganic particles and binder, which is believed to force the inorganic particles together into an agglomerate. The binder is allowed to cool and harden to form the desired granules.
Another suitable method involves heating a fluidized bed of binder, such as HDPE, to a temperature of about 180xc2x0 F. to about 600xc2x0 F., which softens and tackifies the surfaces of the binder particles, and adding inorganic particles, optionally in a different zone of the bed, further heating the mixture to further soften the binder, and cooling to form the desired granules
Another suitable method involves mixing of the binder polymer particles, such as HDPE, and inorganic particles in a mixer, such as a ribbon mixer with high speed shearing, and then compacting the mixture in a continuous compactor to a particle size of about 1 to about 200 mesh. The compacted is then heated from about 30 minutes to about 3 hours at temperatures ranging from about 350xc2x0 to about 600xc2x0 F. The timing of the heating depends on the size of the particle and the amount of the compacting powder in the oven. At a temperature of 350xc2x0 F. to 500xc2x0 F., about 2000 lb of material can be heated for about 6 to about 8 hours.
In another embodiment of he invention, the process is used to agglomerate inorganic particles, such as zirconia or other metal oxide, onto a base or core particle of alumina or carbon, using the polymer particles as a binder. A similar process is followed, involving mixing of the base or core particle with the preheated binder particles, adding the inorganic coating particle, and heat the mixture to soften and at least partially liquefy the binder material, so that it becomes flowable and binds the inorganic coating particles to the base or core particle.
The granules obtained by these processes provide an excellent material for use wherever the inorganic particles can be used, e.g., in fluid purification or gas applications, and where dusting of the granules is undesirable. The granules provide exceptional surface area, because of the small size of the inorganic particles incorporated into them but do not exhibit the breaking characteristics typically encountered with particles in their size range.